1. Field of the Invention
The present invention relates to a control for internal combustion engines and more particularly to an engine control based on engine roughness.
2. Description of the Prior Art
In recent automobile engines, proposals have beem made, for the purpose of improving fuel economy, to detect engine roughness and to operate the engine at an extreme condition where the roughness is just about to occur. For example, in Japanese patent publication 56-33571, published on Aug. 4, 1981, there is disclosed an engine having an intake system including a main intake passage, and an auxiliary air passage for supplying a flow of adjusting air to the main intake passage. The system includes a control arrangement which detects the engine speed fluctuations to discriminate engine roughness and to decrease the flow of adjusting air through the auxiliary air passage, when the engine roughness is detected, to thereby enrich the mixture and suppress the engine roughness. This intake system is considered as being effective to decrease fuel consumption as much as possible without producing unacceptable engine torque fluctuations. Thus, the system can accomplish both riding comfort and fuel economy. Although the Japanese patent publication refers only to an engine roughness control based on the air-fuel ratio of the mixture, a similar control may be made based on other factors, such as ignition timing. For example, the ignition timing may be advanced as much as possible within a limit wherein the engine roughness does not occur.
In the case where the engine roughness control is accomplished through a control of air-fuel ratio, the roughness level increases as the mixture becomes leaner in normal operations so that the control system functions to increase fuel supply, or to decrease the adjusting air supply as taught by the Japanese patent publication, when engine roughness is detected. It should, however, be noted that the engine is operated under certain conditions with a richer mixture. For example, in the engine start and accelerating periods, the mixture to the engine is enriched relative to normal operation, and in this instance, the engine roughness level may be increased as the mixture becomes richer. It will therefore be understood that if the previously described roughness control is performed in a specific engine operating condition such as the engine start or the accelerating period, the air-fuel ratio may be controlled in a direction wherein the engine roughness level further increases. A similar problem will also be produced in the case where the ignition timing is controlled based on the engine roughness.